Mifi unlimited data

Hey guys, I have a prepaid account with verizon. I am paying $35 a month for my galaxy s3 but I don't really need talk or text just unlimited data, even 3g is fine. I kinda know more or less how to get the mifi into my account. However, what I don't know is how to choose the plan , $15/month unlimited data, on their website since there is no option for that. Can somebody please give some hints or explanation on how to accomplish this?
Thanks